A Device-to-Device (D2D) communication network implies a network in which neighboring devices directly perform communication without having to use an infrastructure consisting of a centralized access point such as a Base Station (BS) or an Access Point (AP). In the D2D communication network, a wireless terminal identifies neighboring terminals geographically adjacent to one another, and transmits data by establishing a wireless link with a specific neighboring terminal. As such, since terminals in the D2D communication network perform communication without an aid of other devices, the D2D communication network advantageously can enter a market rapidly without an additional intra structure in comparison with another wireless communication network. In addition, since the D2D communication accommodates data traffics in a localized manner, an overload problem of traffic concentrated at present in the BS or the AP can be solved. Due to such a reason, a standardization organization such as 3rd Generation Partnership Project (3GPP) and Institute of Electrical and Electronics Engineers (IEEE) promotes to establish a D2D communication standard on the basis of Long Term Evolution (LTE)-advanced or Wireless-Fidelity (WiFi). Further, an independent D2D communication technique is under development by companies such as Qualcomm, and the like.
In the D2D communication network, terminals (or devices) must be able to establish a D2D link with another terminal autonomously without an aid of the BS or the AP, and must be able to perform data communication through the D2D link. In this process, a distributed link scheduling method for determining when and which radio resource is used by each of a plurality of D2D links to transmit data is very important in regards to a performance of the D2D communication network. The Qualcomm proposes a distributed link scheduling scheme: X. Wu, S. Tabildar, S. Shakkottai, T. Richardson, J. Li, R. Laroia, and A. Jovicic. FlashLinQ: A synchronous Distributed Scheduler for Peer-to-Peer Ad Hoc Networks, Technical Report, 2010, in consideration of a Signal to Interference Ratio (SIR) between links by exchanging a single-tone discovery signal on the basis of an Orthogonal Frequency Division Multiplexing (OFDM) signal structure in a wireless environment in which terminals are synchronized.
In the FlashLinQ, a medium access status is determined when single-tone discovery signals are exchanged by a Transmit (Tx) terminal and a Receive (Rx) terminal which constitute a D2D link on the basis of a resource unit regionally given according to a Connection IDentifier (CID) of a link in Tx and Rx OFDM blocks in a link scheduling duration. In this process, each Tx terminal calculates an SIR by considering signal interference caused by the Tx terminal itself and expected to have an effect on an Rx terminal of D2D links having a higher priority, and compares the calculated SIR with a threshold. Each Rx terminal calculates an SIR on the basis of signal interference from Tx terminals having a higher priority and compares it with a threshold. In this embodiment, if the SIR calculated by the Tx terminal and the Rx terminal is greater than or equal to the threshold, the Tx and Rx terminals determine to perform data transmission by accessing a medium, and if the calculated SIR is less than the threshold, give up the medium access for links having a higher priority than the Tx and Rx terminals and determine to yield the medium access.
When the medium access status is determined as described above, although Tx/Rx terminals corresponding to a specific D2D link have given up the medium access for a different D2D link having a higher priority than the terminals themselves, there may be a case where the different D2D which is a cause of giving up the medium access actually gives up the medium access for another different D2D link having a higher priority. In this embodiment, since the different D2D link does not actually perform the medium access, an SIR condition of a specific D2D link can satisfy a threshold. However, since the Tx/Rx terminals of the specific D2D link cannot recognize a situation of giving up the medium access of a different D2D link, there may be a situation in which the specific D2D link unnecessarily gives up the medium access.
FIG. 1 illustrates a link scheduling result of a Tx terminal and an Rx terminal which constitute a D2D link according to the conventional technique. Herein, it is assumed a situation in which a first D2D link consisting of a terminal A 101 and a terminal B 102, a second D2D link consisting of a terminal C 103 and a terminal D 104, and a third D2D link consisting of a terminal D 105 and a terminal F 106 determine a medium access status through link scheduling. In addition, it is also assumed that, when a first link of the terminal A 101 and the terminal B 102 which have a highest priority performs a medium access, a second link of the terminal C 103 and the terminal D 104 which have a second priority must perform a yield, and when the second link of the terminal C 103 and the terminal D 104 which have the second priority attempts a medium access, a third link of the terminal D 105 and the terminal F 106 which have a lower priority and which are adjacent to the second link must perform a yield. In such a situation, if the three D2D links simultaneously attempt a medium access, the second link of the terminal C 103 and the terminal D 104 gives up the medium access due to the first link of the terminal A 101 and the terminal B 102 by exchanging a single-tone discovery signal in Tx and Rx OHM blocks, and the third link of the terminal E 105 and the terminal F 106 gives up the medium access due to the second link of the terminal C 103 and the terminal D 104. In this embodiment, although the third link of the terminal E 105 and the terminal F 106 can simultaneously access to a medium in practice together with the first link of the terminal A 101 and the terminal B 102, since it is not known that the second link of the terminal C 103 and the terminal C 104 gives up the medium access, a situation occurs in which the medium access is unnecessarily given up for the second link. Such a problem may result in an unnecessary yield of a plurality of links in the conventional D2D communication network, thereby deteriorating an overall network performance. Further, a medium access of links having a lower priority is unnecessarily limited, thereby deteriorating service quality (e.g., delay, data rate, and the like) of lower links. Accordingly, there is a need to provide a method for avoiding an unnecessary medium access limitation while improving an overall network performance in a D2D communication network which uses distributed link scheduling.